Electromagnetic compatibility is a substantial concern in the design of many electronic devices. Typically, the concern is with preventing radiated and/or conducted emissions generated by a device from exceeding levels prescribed by governmental regulation. Sources of such emissions include, for example, switching power supplies, video signals, clock signals, and other fast transitioning or high frequency signal sources. These sources, however, are considered unintentional radiators, and while they can cause interference with other nearby (or on a common power line in the case of conducted emissions), the signal strength is relatively weak compared to intentional radiators such as radio transmitters.
While considerable effort is taken to ensure that intentional radiating devices operate such that their signals conform to prescribed parameters, a problem arises for devices in close proximity to intentional radiators. The near field can couple into the circuits of nearby electronic devices and interfere with their operation. This includes devices that are substantially identical to the transmitting device. Radio signals can couple into circuitry in nearby devices, inducing voltage and/or current that can interfere with the operation of those circuits. This effect is especially noticed in devices that have data lines and do not constantly refresh data among various components, such as when the device is in a low power state. Data lines tend to have high impedance terminations, which facilitates induced voltages from coupled radio signals that can change the state of information on a given signal or data line. As such, for example, in a portable two-way radio device located in close proximity to another portable two-way radio device that is transmitting, errors can be induced in display segments, keypad-signals, accessory connector interface, and so on.
A transmitting device can be designed to mitigate near field effects of its own antenna since it knows when it is transmitting, but a separate device doesn't have such information. A common solution is to periodically refresh signal lines. However, as portable devices are battery powered, and given the relatively low frequency of occurrence of radio frequency interference events, periodically waking from a low power mode to refresh signal lines represents a largely inefficient operation.
Accordingly, there is a need for a method and apparatus for maintaining signal states in an electronic device that is subject to occasional radio frequency interference without the problems associated with the prior art.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.